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Characteristics of Low-Frequency Seismo-Volcanic Signals

All examples below have been recorded on Soufriere Hills volcano,  Montserrat, West Indies. Low-frequency events have outstanding characteristics  that hold  important clues about  the internal stage of volcanic activity. The fact that they might be used in the future as a forecasting tool  makes them one of our prime research targets.  Below we list some of these characteristics.
 
In contrast to volcano-tectonic events, low-frequency earthquakes have a frequency contents around 1 Hz. Hybrid events differ from long-period events by a small portion of high frequency energy near the onset of the signal (highlighted in blue in time and frequency domain). The
events originate from sources at 1500-2000 m depth.  For comparison see a classic rockfall event.  

 
 
 
 
 
 
 
 
 A common feature is the clustering of low-frequency events into groups or swarms of very similar seismic events. Sometimes these events merge into volcanic tremor. The similarity between events indicates
  • a common source location for all events  within one  swarm ,
  • a non-destructive  repeatable source mechanism, 
  • a feed-back system which allows a constant triggering  of low-frequency events.

 
 
 
 
 
 
 
A radial tilt signal observed on one of the old adjacent dome structures (Chances Peak) shows a remarkable correlation with the occurrence of seismic events. This indicates that seismic signals are linked to the pressurisation process of the volcanic edifice. 

 
 
 
 
 
 
 
 
In this example , continuous tremor precedes an explosion. The seismogramme section  prior to the explosion doesn't reveal any pattern or structure and appears to be just seismic noise. The spectrogramme  however, which shows the spectra of a gliding time window, reveals gliding spectral lines. This can be modelled and interpreted in terms of a pressure increase that changes the gas volume fraction in the magma, hence the seismic velocity and the conduit resonance.
 

In the () you'll find how such a mechanism producing gliding spectral lines can be  modelled and explained.